It is well known on both practical and theoretical grounds that the intensity of light scattered by particles smaller than the wavelength of light is a strong function of particle size. In the Rayleigh Scattering limit where particle radius is below 0.1× wavelength of illumination, the scattered intensity is proportional to particle radius raised to the sixth power and is substantially independent of scattering angle. Above this limit, the angular dependence becomes significant. To describe the scattering in this area, the optical properties of the particle are needed and particle shape also becomes important. The mathematical treatment due to Gustave Mie can be used to predict the scattering from spherical particles in all these regions. A computer program due to Dave is a convenient tool to calculate these effects, and is available as an Appendix to Bohren, C. F. and Huffman, D.R., “Absorption and Scattering of Light by Small Particles”, Wiley, New York, 1983. FIG. 1 shows the scattering of light of 633 nm wavelength from particles from 10 nm to 10 microns of refractive index 1.6 and negligible absorbance (from Bohren & Huffman, supra). It is easy to see that for sizes below 100 nm the scattering angle is relatively unimportant whereas for larger sizes the scattering pattern varies, with the variation being greater at the larger scattering angle.
A computer program that can be used to calculate the scattering for spheres of particular size and refractive index is included as an appendix to Bohren, C. F. and Huffman, D. R., “Absorption and Scattering of Light by Small Particles”, Wiley, New York, 1983. This paper is based on earlier work by J. V. Dave published in 1968. The documents referenced above are all herein incorporated by reference.
Note that at around 1 micron the forward-back scatter ratio is about 104. Hence a light scattering measurement on a sample containing small primary particles and also larger species is very sensitive to small numbers of large particles, or even individual large particles. These unwanted contaminants may be found in all kinds of systems; they may be aggregates of the primary particle or some other material. In the study of virus particles multiplied in a growth medium, for example, whole biological cells or fragments are often present. In light scattering of flowing samples such as the efflux from a size exclusion chromatography system, particle debris from the column itself may be mixed with the molecular species of interest.
The use of simultaneous multiple detectors to analyze the intensity distribution of light scattering from a population of particles has been widely used and is known as Static Light Scattering' (SLS); when conducted using a planar array of detectors in the focal plane of a lens or equivalent optical system so that the light that is detected is in a forward direction over a limited angular range the term ‘laser diffraction’ is often applied. Hybrid systems combining forward, side, and backscatter, such as the Mastersizer 2000, are available, for example, from Malvern Instruments of Malvern UK.